We were sometimes driving 97 & 16 on long weekends.
The provincial campground at Lac La Hache always had spots, even later in the evening. That was usually our first stop when driving north from Vancouver Island. Even on a long weekend, it had some spots left over.
If you feel like doing more time on that day, you could try for Ten Mile Lake provincial park campground (still on Hwy 97) BUT it is a popular spot on weekends. There is a bit of an overflow camping area, but I expect on a long weekend it would be busy too. We often stopped at the day-use area for lunch.
Beaumont provincial park campground on Hwy 16 was full the two times we tried to stay there on long weekends. It is a bit busier because it is lakeside. We did find spots at Peterson's Beach rec site on the north side of the lake though. It is rustic but do-able. Sites are smallish and rough though.
Tyhee Lake provincial park near Smithers is busy on long weekends. Popular spot. It is large though. Takes reservations too but might have a minimum stay because it is a long weekend.
Red Bluff provincial park campground, near Topley & Granisle was a real nice stay. Bit of a drive in but not too far. We were the only ones there!
Seeley Lake, between Kitwanga and New Hazelton is very close to the highway and often busy. It's a small campground. Never stayed there because it was always full.
Once stayed at Dave's RV Park in Vanderhoof. Very nice spot. Well-maintained. About a kilometre off the highway.
Now that we live in Yukon, no worries about camping along Hwy 97 & 16!